Method for connecting optical fibers and connection structure of optical fibers

ABSTRACT

A method for connecting optical fibers and a connection structure of optical fibers capable of suppressing axial misalignment between cores in end-to-end connection of optical fibers at least one of which has a clad for a non-circular shape. A core of at least one optical fiber has a circular shape and a clad thereof has a non-circular shape, and in the optical fiber having the clad of the non-circular shape, the clad is formed to have a more circular shape at and near a splice than at another portion thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.13/052,679, filed on Mar. 21, 2011, which claims the benefit of priorityfrom the prior Japanese Patent Application No. 2010-071453, filed onMar. 26, 2010, the entire contents of which are incorporated herein byreferences.

TECHNICAL FIELD

The invention relates to a method for connecting optical fibers and aconnection structure of optical fibers.

BACKGROUND ART

Fiber laser devices can produce a small beam spot of light having highfocusing performance and high power density, and process in a noncontactmanner. Accordingly, fiber laser devices are used in various fields suchas the laser processing field and the medical field. A fiber laserdevice includes an amplification optical fiber having a core coated witha clad and doped with an active element for amplifying light. However,it is known that skew mode propagation may occur in the amplificationoptical fiber, where part of pumping light propagates only through theclad without being absorbed by the active element and does notcontribute to amplification of light. One known technique for preventingsuch skew mode propagation is to form the clad of the amplificationoptical fiber to have a cross-section of a non-circular shape such as aD-shape or a polygonal shape.

Patent Document 1 listed below discloses an amplification optical fiberin which a clad is formed to have a polygonal cross-section as mentionedabove. According to Patent Document 1, in connecting the amplificationoptical fiber having a clad of a polygonal shape to another opticalfiber, cores of both optical fibers to be connected are butted againsteach other under side view observation, where accurate positions of thecores may not be visually obtained depending on the shape of the clad ofthe amplification optical fiber. Patent Document 1 also states thataccurate positions of the cores can be visually obtained by side viewobservation if the clad of the amplification optical fiber has asubstantially square shape.

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    2003-229617

SUMMARY OF THE INVENTION

However, even if the clad of the amplification optical fiber has asubstantially square shape, there is a disadvantage that refraction oflight passing from along side faces of the clad to the core changes whenthe amplification optical fiber is turned about the axis, which makes itdifficult to obtain accurate positions of the cores by side viewobservation. Therefore, there is a disadvantage that axial misalignmentbetween cores is likely to occur if optical fibers at least one of whichhas a clad of a non-circular shape are connected end-to-end to eachother under side view observation.

Therefore, an object of the invention is to provide a method forconnecting optical fibers and a connection structure of optical fiberscapable of suppressing axial misalignment between cores in end-to-endconnection of optical fibers at least one of which has a clad of anon-circular shape.

A method for connecting optical fibers according to the inventionincludes: a preparing step of preparing a pair of optical fibersincluding a first optical fiber having a clad of a non-circular shapeand a second optical fiber having a clad of a circular shape; a formingstep of forming the clad of the first optical fiber at and near one endthereof to have a more circular shape from the non-circular shape; analigning step of observing in side view a formed portion of the clad ofthe first optical fiber and around one end of the second optical fiberand aligning a core of the first optical fiber at and near the one endthereof and a core of the second optical fiber at and near the one endthereof in a straight line; and a fusing step of butting the one end ofthe first optical fiber and the one end of the second optical fiberagainst each other and fusing the ends together.

According to such a method for connecting optical fibers, the clad isformed to have a more circular shape at and near the one end of thefirst optical fiber having the clad of a non-circular shape, and thepair of optical fibers is then aligned under side view observation. Itis therefore possible to prevent a change in refraction of light passingfrom along side faces of the clad to the core even if the amplificationoptical fiber is turned about the axis during side view observation. Asa result, axial misalignment between the cores can be suppressed in sideview observation and the one end of the first optical fiber and the oneend of the second optical fiber can be connected to each other in astate where axial misalignment is suppressed.

Alternatively, a method for connecting optical fibers according to theinvention includes: a preparing step of preparing a pair of opticalfibers each having a clad of a non-circular shape; a forming step offorming the clad of each of the optical fibers at and near one endthereof to have a more circular shape from the non-circular shape; analigning step of observing in side view formed portions of the clads ofthe optical fibers and aligning a core of one optical fiber at and nearthe one end thereof and a core of another optical fiber near the one endthereof in a straight line; and a fusing step of butting the one ends ofthe optical fibers against each other and fusing the ends together.

According to such a method for connecting optical fibers, the respectiveclads are formed to have a more circular shape at and near the one endsof the pair of optical fibers each having the clad of a non-circularshape, and the pair of optical fibers is then aligned under side viewobservation. It is therefore possible to prevent a change in refractionof light passing from along side faces of the clads to the cores even ifthe optical fibers are turned about the axis during side viewobservation. As a result, axial misalignment between the cores can besuppressed in side view observation and the one ends of the respectiveoptical fibers can be connected to each other in a state where axialmisalignment is suppressed.

In the forming step of the method for connecting optical fibersdescribed above, the clad is preferably formed for 100 μm or longer in alongitudinal direction of the optical fiber from the one end thereof.

Forming of the clad for 100 μm or longer in the longitudinal directionof the optical fiber from the one end thereof facilitates side viewobservation at a plurality of points in the aligning step.

In the method for connecting optical fibers described above, the formingin the forming step is preferably carried out by discharge heating.

Since a fusion splicer for connecting optical fibers can be generallyused for discharge heating without any change, this can save the troubleof providing an additional device for the forming.

In the method for connecting optical fibers described above, thedischarge heating is preferably carried out by intermittent discharge.

Such discharge heating carried out by intermittent discharge allows heatto be less likely to be conducted to the core and prevents temperaturerise of the core. Thus, the clad can be formed while deformation of thecore by heat is prevented. Therefore, a splice loss of light propagatingthrough the core after the connection can be suppressed.

In the method for connecting optical fibers described above, anon-discharge time is preferably longer than a discharge time in theintermittent discharge.

Such intermittent discharge allows heat to be much less likely to beconducted to the core and prevents temperature rise of the core.

In the method for connecting optical fibers described above, thenon-discharge time is more preferably four times or more longer than thedischarge time.

Such intermittent discharge allows heat to be still less likely to beconducted to the core and prevents temperature rise of the core.

A connection structure of optical fibers according to the invention is aconnection structure of a pair of optical fibers that are connectedend-to-end, wherein a clad of at least one optical fiber has anon-circular shape, and in the optical fiber having the clad of anon-circular shape, the clad is formed to have a more circular shape atand near a splice than at another portion thereof.

According to such a connection structure of optical fibers, axialmisalignment between the cores at the splice can be suppressed and asplice loss of light propagating through the core can be suppressed. Itis generally known that a portion connected by heating is more fragilethan the other portion since the glass strength is decreased by thermalstrain applied to the glass. If an optical fiber in which a clad is notsubjected to forming and the clad has a non-circular shape is used, astress applied to or near a splice is not distributed uniformly at theouter periphery of the clad at and near the splice but causes stressconcentration at a portion thereof. Optical fibers are easily broken atthe portion. In contrast, according to the connection structure ofoptical fibers according to the invention, the clad at and near thesplice is formed to have a more circular shape. Accordingly, a stressapplied to or near the splice can be distributed substantially uniformlyover the entire outer periphery and breaking can be suppressed ascompared to the case where a clad at and near a splice is not subjectedto forming. For example, in a case where a clad has a polygonal shape, astress applied to a splice causes stress concentration at a corneredportion of an outer circumferential face at and near the splice and afracture of the fiber is likely to occur at the cornered portion. Incontrast, according to the connection structure of optical fibers of theinvention, the clad is formed to have a more circular shape at and nearthe splice, whereby corner portions of the outer circumferential face ofthe clad are eliminated. As a result, even if a stress is applied to thesplice, the stress can be distributed substantially uniformly over theentire outer periphery and breaking can be suppressed as compared to thecase where a clad at a splice is not subjected to forming.

In the connection structure of optical fibers described above, in theoptical fiber having the clad of a non-circular shape, the clad isformed preferably for 100 μm or longer in a longitudinal direction fromthe splice.

With such a connection structure of optical fibers, a splice loss oflight caused by axial misalignment between cores can be suppressed.Moreover, a zone where the thermal strain described above occurssignificantly is a zone which heat caused by electric discharge reachesand is at most within 100 μm from the connecting point in a typicalfusion splicer. Therefore, since cornered portions of a polygonal shapeor other non-circular shapes are eliminated within 100 μm from theconnecting point by forming for a length of 100 μm or longer, there isno portion where the stress concentrates at the connecting point, and itis thus possible to suppress breaking at the connecting point.

As described above, a method for connecting optical fibers and aconnection structure of optical fibers capable of preventing axialmisalignment between cores are provided according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a fiber laser device having a connectionstructure of optical fibers according to an embodiment of the invention.

FIG. 2 is a view showing a structure of a cross-section perpendicular toa longitudinal direction of an amplification optical fiber shown in FIG.1.

FIG. 3 is a view showing a structure of a cross-section perpendicular toa longitudinal direction of a delivery fiber shown in FIG. 1.

FIG. 4 is a view showing a connection between the amplification opticalfiber and the delivery fiber shown in FIG. 1.

FIG. 5 is a flowchart showing steps of a method for connecting theamplification optical fiber and the delivery fiber shown in FIG. 4.

FIG. 6 is a view showing the amplification optical fiber after astripping step.

FIG. 7 is a view showing the amplification optical fiber after a formingstep.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of a method for connecting optical fibers and aconnection structure of optical fibers according to the invention willbe described hereinafter referring to the drawings.

FIG. 1 is a diagram showing a fiber laser device having a connectionstructure of optical fibers according to an embodiment of the invention.As shown in FIG. 1, a laser device 1 is a fiber laser device including,as main components: a seed light source 10 configured to output seedlight; a pumping light source 20 configured to output pumping light; anamplification optical fiber (first optical fiber) 30 to which the seedlight and the pumping light are input; a combiner 40 configured toconnect the seed light source 10 and the pumping light source 20 to theamplification optical fiber 30; and a delivery fiber (second opticalfiber) 50 one end of which is connected to the amplification opticalfiber 30.

The seed light source 10 may be constituted by a laser light sourceincluding a laser diode or by a fiber laser device of fabry-perot typeor fibering type, for example. The seed light output from the seed lightsource 10 may be laser light having a wavelength of 1070 nm, forexample, but is not particularly limited thereto. The seed light source10 is connected to a fiber 15 for propagation of seed light having acore and a clad coating the core. Seed light output from the seed lightsource 10 propagates through the core of the fiber 15 for propagation ofseed light. The fiber 15 for propagation of seed light may be asingle-mode fiber, for example, in which case the seed light propagatesas single-mode light through the fiber 15 for propagation of seed light.

The pumping light source 20 is constituted by a plurality of laserdiodes 21, and configured to output pumping light having a wavelength of915 nm, for example, when the seed light has a wavelength of 1070 nm asdescribed above. The laser diodes 21 of the pumping light source 20 areconnected to fibers 22 for propagation of pumping light, respectively.Pumping light output from each laser diode 21 propagates through thecorresponding fiber 22 for propagation of pumping light. The fiber 22for propagation of pumping light may be a multi-mode fiber, for example,in which case the pumping light propagates as multi-mode light throughthe fiber 22 for propagation of pumping light.

FIG. 2 is a view showing a structure of a cross-section perpendicular tothe longitudinal direction of the amplification optical fiber 30. Asshown in FIG. 2, the amplification optical fiber 30 has a core 31, aclad 32 coating the core 31, a plastic clad 33 coating the clad 32, anda coating layer 34 coating the plastic clad 33. The core 31 has acircular shape and the clad 32 has a non-circular shape in across-section of the amplification optical fiber 30. In addition, theplastic clad 33 and the coating layer 34 have circular shapes. In thisembodiment, the clad 32 has a septagonal cross-sectional shape.

The clad 32 has a lower refractive index than the core 31, and theplastic clad 33 has a further lower refractive index than the clad 32. Amaterial for the core 31 may be silica doped with an element such asgermanium that increases the refractive index and an active element suchas ytterbium (Yb) that is pumped by pumping light output from thepumping light source 20, for example. Such an active element may be arare earth element, examples of which include thulium (Tm), cerium (Ce),neodymium (Nd), europium (Eu) and erbium (Er) in addition to Yb.Examples of the active element also include bismuth (Bi) or the like inaddition to the rare earth element. A material for the clad 32 may bepure silica without any dopant, for example. A material for the plasticclad 33 may be UV curable resin, for example, and a material for thecoating layer 34 may be UV curable resin different from that for theplastic clad 33, for example.

The combiner 40 connects the fiber 15 for propagation of seed light andthe respective fibers 22 for propagation of pumping light to theamplification optical fiber 30. Specifically, the core of the fiber 15for propagation of seed light is connected end-to-end to the core 31 ofthe amplification optical fiber 30 in the combiner 40. Further, cores ofthe respective fibers 22 for propagation of pumping light are connectedend-to-end to the clad 32 in the combiner 40. Accordingly, seed lightoutput from the seed light source 10 is input to the core 31 of theamplification optical fiber 30, and pumping light output from thepumping light source 20 is input to the clad 32 of the amplificationoptical fiber 30.

FIG. 3 is a view showing a structure of a cross-section perpendicular tothe longitudinal direction of the delivery fiber 50 shown in FIG. 1. Asshown in FIG. 3, the delivery fiber 50 has a core 51, a clad 52 coatingthe core 51, a plastic clad 53 coating the clad 52, and a coating layer54 coating the plastic clad 53. All of the core 51, the clad 52, theplastic clad 53 and the coating layer 54 have circular shapes in across-section of the delivery fiber 50.

The clad 52 has a lower refractive index than the core 51, and theplastic clad 53 has a further lower refractive index than the clad 52. Amaterial for the core 51 may be silica doped with an element such asgermanium that increases the refractive index, for example, and amaterial for the clad 52 may be pure silica without any dopant, forexample. A material for the plastic clad 53 may be UV curable resin, forexample, and a material for the coating layer 54 may be UV curable resindifferent from that for the plastic clad 53, for example.

FIG. 4 is a view showing the connection between the amplificationoptical fiber 30 and the delivery fiber 50 shown in FIG. 1. As shown inFIG. 4, one end 35 of the amplification optical fiber 30 and one end 55of the delivery fiber 50 are connected end-to-end at a splice 60.Specifically, the plastic clad 33 and the coating layer 34 of theamplification optical fiber 30 are stripped off for a predeterminedlength from the end 35. The clad 32 at and near the end 35 of theamplification optical fiber 30 is formed to have a substantiallycircular cross-sectional shape in a cross-section perpendicular to thelongitudinal direction of the amplification optical fiber 30 for alength L from the end 35. The plastic clad 53 and the coating layer 54of the delivery fiber 50 are also stripped off for a predeterminedlength from the end 55. The end 35 of the amplification optical fiber 30and the end 55 of the delivery fiber 50 are connected end-to-end in astate where the core 31 at and near the end 35 of the amplificationoptical fiber 30 and the core 51 at and near the end 55 of the deliveryfiber 50 are aligned with each other in a straight line.

The length L for which the clad 32 of the amplification optical fiber 30is formed to have a substantially circular shape is preferably 100 μm orlonger in terms of preventing axial misalignment between the core 31 andthe core 51.

Next, a method for connecting the amplification optical fiber 30 and thedelivery fiber 50 shown in FIG. 4 will be described.

FIG. 5 is a flowchart showing steps of the method for connecting theamplification optical fiber 30 and the delivery fiber 50 shown in FIG.4. As shown in FIG. 5, the method for connecting the amplificationoptical fiber 30 and the delivery fiber 50 includes: a preparing step S1of preparing the amplification optical fiber 30 and the delivery fiber50; a stripping step S2 of stripping the plastic clads 33 and 53 and thecoating layers 34 and 54 for a predetermined length from the end 35 ofthe amplification optical fiber 30 and for a predetermined length fromthe end 55 of the delivery fiber 50, respectively; a forming step S3 offorming the clad 32 at and near the end 35 of the amplification opticalfiber 30 to have a more circular shape; an aligning step S4 of aligningthe core 31 at and near the end 35 of the amplification optical fiber 30and the core 51 at and near the end 55 of the delivery fiber 50 witheach other in a straight line under side view observation; and a fusingstep S5 of butting the end 35 of the amplification optical fiber 30 andthe end 55 of the delivery fiber 50 against each other and fusing theends together.

(Preparing Step S1)

First, the amplification optical fiber 30 and the delivery fiber 50 areprepared. Thus, a pair of a first optical fiber having a clad with anon-circular shape and a second optical fiber having a clad with acircular shape is prepared.

(Stripping Step S2)

Next, the plastic clad 33 and the coating layer 34 of the preparedamplification optical fiber 30 are stripped off for a predeterminedlength from the end 35 to expose the clad 32. In this step, the plasticclad 33 and the coating layer 34 are stripped off so that the clad 32 isexposed for a length longer than the length L for which the shape of theclad 32 is to be formed as shown in FIG. 4. As a result, the clad 32 ofthe amplification optical fiber 30 is exposed for the predeterminedlength form the end 35 as shown in FIG. 6. Similarly, the plastic clad53 and the coating layer 54 of the delivery fiber 50 are stripped offfor a predetermined length from the end 55 to expose the clad 52.

(Forming Step S3)

Next, the clad 32 at and near the end 35 of the amplification opticalfiber 30 is formed to have a more circular cross-sectional shape in across-section perpendicular to the longitudinal direction of theamplification optical fiber 30. In this embodiment, the clad 32 isformed to have a substantially circular shape. The forming can becarried out by hot forming using electric discharge, a flame burner, alaser or the like, etching using chemicals, mechanical forming bypolishing, or the like.

In hot forming, the clad 32 is formed to have a more circularcross-sectional shape by the action of surface tension of the clad 32melted by heating. In hot forming using discharge heating, heating byintermittent discharge is preferable. With the intermittent discharge,heat is less likely to be conducted to the core 31 of the amplificationoptical fiber 30, and the clad 32 can thus be formed while preventingthe core 31 from being deformed by heat. Therefore, a splice loss oflight propagating through the core 31 after connection can besuppressed. More preferably, in forming the clad 32 by intermittentdischarge heating, the non-discharge time is longer than the dischargetime so that heat is much less likely to be conducted to the core. Stillmore preferably, the non-discharge time during the intermittentdischarge is four times or more longer than the discharge time so thatheat is still less likely to be conducted to the core. In hot formingusing a flame burner, an oxyhydrogen burner may be used, for example. Inhot forming using a laser, a CO₂ laser may be used, for example.

In etching or mechanical forming, the clad 32 is formed to have a morecircular cross-sectional shape by etching or polishing corners in thecross-sectional shape of the clad 32. In etching using chemicals,hydrofluoric acid (HF) may be used for etching, for example. Inmechanical forming, a glass abrasive may be used for polishing, forexample.

In this manner, the clad 32 of the amplification optical fiber 30becomes in a state formed to have a substantially circularcross-sectional shape at and near the end 35 as shown in FIG. 7.

The length L for which the cross-sectional shape of the clad 32 isformed is preferably 100 μm or longer from the end 35 of theamplification optical fiber 30. Such a length allows easier alignment ofthe core 31 of the amplification optical fiber 30 and the core 51 of thedelivery fiber 50 with each other in a straight line in the aligningstep S4.

(Aligning step S4)

Next, the end 35 of the amplification optical fiber 30 and the end 55 ofthe delivery fiber 50 are arranged to face each other, and the core 31at and near the end 35 of the amplification optical fiber 30 and thecore 51 at and near the end 55 of the delivery fiber 50 are aligned witheach other in a straight line. In this step, the core 31 of theamplification optical fiber 30 and the core 51 of the delivery fiber 50are aligned with each other under side view observation. In the sideview observation of the amplification optical fiber 30, a portion wherethe clad 32 is formed is observed. The side view observation ispreferably conducted at a plurality of points. In this case, the lengthL for which the cross-sectional shape of the clad 32 is formed ispreferably 100 μm or longer from the end 35 of the amplification opticalfiber 30, which allows easier observation at a plurality of points andeasier alignment of the core 31 of the amplification optical fiber 30and the core 51 of the delivery fiber 50 with each other in a straightline as described above.

(Fusing Step S5)

Next, as described above, the end 35 of the amplification optical fiber30 and the end 55 of the delivery fiber 50 are butted against each otherand fused together to be connected end-to-end. The fusing may be fusingby means of electric discharge, fusing a flame burner, fusing a laser,or the like. Fusing by discharge heating may be carried out bycontinuous discharge or intermittent discharge. In fusing a flameburner, an oxyhydrogen burner may be used, for example. In hot formingusing a laser, a CO₂ laser may be used, for example. The end 35 of theamplification optical fiber 30 and the end 55 of the delivery fiber 50are thus connected end-to-end, whereby the amplification optical fiber30 and the delivery fiber 50 are connected as shown in FIG. 4.

According to the method for connecting optical fibers of this embodimentas described above, the clad 32 having a non-circular shape is formed tohave a more circular shape at and near the end 35 of the amplificationoptical fiber 30, and the amplification optical fiber 30 and thedelivery fiber 50 are then aligned with each other under side viewobservation. Therefore, even if the amplification optical fiber 30 turnsabout the axis during side view observation, change in refraction oflight passing from along side faces of the clad 32 to the core can beprevented. Therefore, axial misalignment between the core 31 and thecore 51 can be prevented under side view observation, and the end 35 ofthe amplification optical fiber 30 and the end 55 of the delivery fiber50 can be connected in a state where axial misalignment is suppressed.

According to the connection structure of optical fibers of thisembodiment, axial misalignment between the core 31 and the core 51 atthe splice therebetween can be prevented. Therefore, a splice loss oflight propagating through the core 31 to the core 51 can be prevented.In addition, since the clad 32 at the splice is formed to have a morecircular shape, a stress applied to the splice, if any, can bedistributed, and the amplification optical fiber 30 and the deliveryfiber 50 can be prevented from breaking at the splice.

Although the invention has been described above by reference to acertain embodiment as an example, the invention is not limited thereto.

For example, the clad 32 is formed to have a substantially circularcross-sectional shape at and near the end 35 of the amplificationoptical fiber 30 that is the first optical fiber in the embodiment, butthe invention is not limited thereto as long as the clad 32 is formed tohave a more circular cross-sectional shape. Also in this case, axialmisalignment between the core 31 of the amplification optical fiber 30and the core 51 of the delivery fiber 50 can be better prevented in thealigning step S4 as compared to a case where the clad 32 is notsubjected to forming.

In the embodiment described above, the clad 32 has a septagonal shape ina cross-section perpendicular to the longitudinal direction of theamplification optical fiber 30. However, the shape of the clad 32 may beany of other polygonal shapes, or a non-circular shape such as a D-shapeor an elliptical shape.

In the embodiment, the clad 32 of the amplification optical fiber 30that is the first optical fiber has a non-circular shape and the clad 52of the delivery fiber 50 that is the second optical fiber has a circularshape. However, the invention is not limited thereto, and may be appliedto a connection structure and a method for connecting a pair of opticalfibers having clads of non-circular shapes in which a clad 52 of adelivery fiber 50 also has a non-circular shape. In this case, anamplification optical fiber 30 that is the first optical fiber (oneoptical fiber) and a delivery fiber 50 that is the second optical fiber(another optical fiber) each having a clad with a non-circular shape areprepared in the preparing step S1. Next, plastic clads 33 and 53 andcoating layers 34 and 54 are stripped off for a predetermined lengthfrom one end 35 of the amplification optical fiber 30 and from one end55 of the delivery fiber 50 in the stripping step S2. Then, the clad 32of the amplification optical fiber 30 is formed in the same manner as inthe embodiment, and the clad 52 is formed to have a more circular shapeat and near the end 55 of the delivery fiber 50 in the forming step S3.The forming of the clad 52 may be in the same manner as the forming ofthe clad 32 at and near the end 35 of the amplification optical fiber30. The amplification optical fiber 30 is observed in the same manner asin the embodiment, and the delivery fiber 50 is under side viewobservation at a portion of the clad 52 formed in the aligning step S4.Therefore, also in the delivery fiber 50, the length for which thecross-sectional shape of the clad 52 is formed is preferably 100 μm orlonger from the end 55 of the delivery fiber 50, which allows easierobservation at a plurality of points and easier alignment of the core 31of the amplification optical fiber 30 and the core 51 of the deliveryfiber 50 with each other in a straight line. Next, the fusing step S5 iscarried out in the same manner as the embodiment.

Moreover, for example, the connection structure and the method forconnecting the amplification optical fiber 30 that is the first opticalfiber and the delivery fiber 50 that is the second optical fiber havebeen described in the embodiment. However, the invention is not limitedthereto, and may be applied to a connection structure and a method forconnecting other types of optical fibers used as the first optical fiberand the second optical fiber to each other.

If the plastic clad and the coating layer at and near one end of theprepared optical fiber are already stripped off in the preparing stepS1, the stripping step S2 is not needed.

EXAMPLES

The invention will be more specifically explained below with examplesand a comparative example, but the invention is not limited thereto.

Example 1

A double-clad fiber having a core, a clad coating the core, a plasticclad coating the clad and a coating layer coating the plastic clad, anda single-mode fiber having a core, a clad coating the core and a coatinglayer coating the clad were prepared. In the double-clad fiber, the corehad a diameter of 7 μm and the clad had a septagonal shape in across-section perpendicular to the longitudinal direction of the fiberwith an average outer diameter of 120 μm. In the single-mode fiber, onthe other hand, the core had a diameter of 8 μm and the clad had acircular shape in a cross-section perpendicular to the longitudinaldirection of the fiber with an outer diameter of 125 μm.

Next, the plastic clad and the coating layer of the double-clad fiberwas stripped off for 10 mm from one end thereof. In addition, thecoating layer of the single-mode fiber was stripped off for 10 mm fromone end thereof.

Next, the double-clad fiber and the single-mode fiber were arranged in afusion splicer. As the fusion splicer, FSM-40F manufactured by FujikuraLtd. was used.

Next, discharge heating was applied to one end of the double-clad fiberat a discharge current of 12 mA for a discharge time of 50 μs. Then,discharge was temporarily stopped, the double-clad fiber was moved 100μm relative to discharge electrodes, and discharge heating was appliedagain to an inner portion at 100 μm in the longitudinal direction fromthe end of the double-clad fiber at equal power and for equal dischargetime. The clad of the double-clad fiber was formed in this manner tohave a more circular shape than the septagonal shape for 100 μm from theend.

Next, the formed portion of the clad of the double-clad fiber and theformed portion of the single-mode fiber where the coating layer wasstripped off were aligned with each other under side view observation.Then, the end of the double-clad fiber and the end of the single-modefiber were butted against each other and fused together.

Examples 2 to 5

Next, the double-clad fiber and the single-mode fiber were connected inthe same manner as Example 1 except that the discharge time of dischargeheating applied to the end of the double-clad fiber and dischargeheating applied to the inner portion at 100 μm in the longitudinaldirection from the end thereof was as shown in Table 1.

Comparative Example 1

The double-clad fiber and the single-mode fiber were connected in thesame manner as Example 1 except that the double-clad fiber was notsubjected to forming by discharge heating.

Next, signal light was caused to propagate through the core of thedouble-clad fiber to the core of the single-mode fiber of each ofExamples 1 to 5 and Comparative Example 1. The wavelength of the signallight was 1070 μm. The resulting splice loss of the signal light at thesplice of the double-clad fiber and the single-mode fiber was as valuesshown in Table 1.

TABLE 1 Discharge time (μs) Splice loss (dB) Example 1 50 2.82 Example 2100 0.82 Example 3 200 0.32 Example 4 300 0.50 Example 5 400 1.56Comparative 0 3.52 Example 1

As shown in Table 1, the result shows that the splice loss was less inExamples 1 to 5 as compared to Comparative Example 1. This can beattributed to the fact that axial misalignment between the cores of thedouble-clad fiber and the single-mode fiber was prevented in alignmentof the double-clad fiber and the single-mode fiber as a result offorming the clad of the double-clad fiber to have a more circular shapeand the double-clad fiber and the single-mode fiber were connected in astate where axial misalignment was thus suppressed. In Examples 2, 3 and4, the splice loss was less than 1 dB, which can be attributed to thefact that the clad was formed to have a more circular shape. Therefore,it was found that a discharge time of 100 to 300 μs is more preferablefor an optical fiber in which a clad has a septagonal shape with anaverage outer diameter of 120 μm.

Examples 6 to 10

The double-clad fiber and the single-mode fiber were connected to eachother in the same manner as Example 1 except that electric dischargeapplied to the end of the double-clad fiber and electric dischargeapplied to the inner portion at 100 μm in the longitudinal directionfrom the end were intermittent discharge, where the discharge time andthe non-discharge time were as shown in Table 2 and the ratio of thedischarge time to the non-discharge time was as shown in Table 2, andthat the electric discharge was repeated ten times.

Next, signal light was caused to propagate through the core of thedouble-clad fiber to the core of the single-mode fiber of each ofExamples 6 to 10. The wavelength of the signal light was 1070 μm. Theresulting splice loss of the signal light at the splice of thedouble-clad fiber and the single-mode fiber was as values shown in Table2.

TABLE 2 Ratio (Discharge Splice Discharge Non-Dischargetime:Non-dischrage Loss time (μs) time (μs) time) (dB) Example 6 50 501:1 2.66 Example 7 50 100 1:2 1.87 Example 8 50 200 1:4 0.66 Example 950 300 1:6 0.12 Example 10 50 400 1:8 0.14

Examples 11 to 15

The double-clad fiber and the single-mode fiber were connected to eachother in the same manner as Example 1 except that a double-clad fiber inwhich a clad has a septagonal shape with an average outer diameter of400 μm was used, that electric discharge applied to the end of thedouble-clad fiber and electric discharge applied to the inner portion at100 μm in the longitudinal direction from the end were intermittentdischarge, where the discharge time and the non-discharge time were asshown in Table 3 and the ratio of the discharge time to thenon-discharge time was as shown in Table 3, and that the electricdischarge was repeated ten times.

Next, signal light was caused to propagate through the core of thedouble-clad fiber to the core of the single-mode fiber of each ofExamples 11 to 15. The wavelength of the signal light was 1070 μm. Theresulting splice loss of the signal light at the splice of thedouble-clad fiber and the single-mode fiber was as values shown in Table3.

TABLE 3 Ratio (Discharge Splice Discharge Non-Dischargetime:Non-dischrage Loss time (μs) time (μs) time) (dB) Example 11 50 501:1 2.25 Example 12 50 100 1:2 1.56 Example 13 50 200 1:4 0.88 Example14 50 300 1:6 0.18 Example 15 50 400 1:8 0.16

The results in Tables 2 and 3 show that the splice loss was less than 1dB in Examples 8, 9 and 10 and Examples 13, 14 and 15, and particularlyless than 0.2 dB in Examples 9 and 10 and Examples 14 and 15. Therefore,in intermittent discharge, it is preferable that the non-discharge timebe four times or more longer than the discharge time, with which thedouble-clad fiber and the single-mode fiber are connected in a statewhere axial misalignment is suppressed regardless of the outer diameterof the clad of the optical fiber that is formed. It is furtherpreferable that the non-discharge time be six times or more longer thanthe discharge time, with which the double-clad fiber and the single-modefiber are connected in a state where axial misalignment is furthersuppressed.

INDUSTRIAL APPLICABILITY

According to the invention, a method for connecting optical fibers and aconnection structure of optical fibers capable of suppressing axialmisalignment between cores are provided.

DESCRIPTION OF REFERENCE NUMERALS

-   1 . . . laser device-   10 . . . seed light source-   15 . . . fiber for propagation of seed light-   20 . . . pumping light source-   21 . . . laser diode-   22 . . . fiber for propagation of pumping light-   30 . . . amplification optical fiber-   31 . . . core-   32 . . . clad-   33 . . . plastic clad-   34 . . . coating layer-   40 . . . combiner-   50 . . . delivery fiber-   51 . . . core-   52 . . . clad-   53 . . . plastic clad-   54 . . . coating layer-   60 . . . splice point-   S1 . . . preparing step-   S2 . . . stripping step-   S3 . . . forming step-   S4 . . . aligning step-   S5 . . . fusing step

What is claimed is:
 1. A connection structure of a pair of opticalfibers that are connected end-to-end, wherein a core of at least oneoptical fiber has a circular shape and a clad thereof has a non-circularshape, and in the optical fiber having the clad of the non-circularshape, the clad is formed to have a more circular shape at and near asplice than at another portion thereof.
 2. The connection structure ofoptical fibers according to claim 1, wherein in the optical fiber havingthe clad of the non-circular shape, the clad is formed for 100 μm orlonger in a longitudinal direction from the splice.